SharingSugar wrote:If anyone can show me historical evidence ..., I will read it gladly and admit I'm wrong.

They also thought the ratio of free state to slave state was stable. They didn't plan on new states.

New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.Article IV, Sec. 3.

This imbalance of trade is what caused the Civil War.

Between 1803 and 1854, the United States achieved a vast expansion of territory through purchase, negotiation and conquest.[62] Of the states carved out of these territories by 1845, all had entered the union as slave states: Louisiana, Missouri, Arkansas, Florida and Texas, as well as the southern portions of Alabama and Mississippi.[63] And with the conquest of northern Mexico, including California, in 1848, slaveholding interests looked forward to the institution flourishing in these lands as well. Southerners also anticipated garnering slaves and slave states in Cuba and Central America.[63][64] Northern free soil interests vigorously sought to curtail any further expansion of slave soil. It was these territorial disputes that the proslavery and antislavery forces collided over.[65][66]

The existence of slavery in the southern states was far less politically polarizing than the explosive question of the territorial expansion of the institution in the west.[67] Moreover, Americans were informed by two well-established readings of the Constitution regarding human bondage: that the slave states had complete autonomy over the institution within their boundaries, and that the domestic slave trade – trade among the states – was immune to federal interference.[68][69] The only feasible strategy available to attack slavery was to restrict its expansion into the new territories.[70] Slaveholding interests fully grasped the danger that this strategy posed to them.[71][72] Both the South and the North believed: “The power to decide the question of slavery for the territories was the power to determine the future of slavery itself.”[73][74]

With the emergence of the Republicans as the nation's first major sectional party by the mid-1850s, politics became the stage on which sectional tensions were played out. Although much of the West– the focal point of sectional tensions– was unfit for cotton cultivation, Southern secessionists read the political fallout as a sign that their power in national politics was rapidly weakening.

Once the election returns were certain, a special South Carolina convention declared "that the Union now subsisting between South Carolina and other states under the name of the 'United States of America' is hereby dissolved", heralding the secession of six more cotton states by February, and the formation of an independent nation, the Confederate States of America.